Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC stack assembly is preferably fueled by “reformate” gas, which is the effluent from a catalytic gasoline oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 900° C.
A complete fuel cell stack assembly comprises a plurality of components and sub-assemblies joined together mechanically to provide the desired flow paths and control pathways for the liquid hydrocarbon, reactive gases, spent gases, and cooling gases. It is essential that the joints or interfaces between the components and sub-assemblies be durably leak-free at temperatures from below O° C. to as high as at least 900° C., at pressures from subatmospheric to up to several atmospheres. Such conditions place very high demands on materials selected for gaskets at these joints and interfaces.
It is known to use various glass and ceramic compositions as sealants. These sealants are also useful as dielectric insulators between adjacent cell elements, such as for example anode plates and cathode plates, which operate at different voltage potentials. However, a drawback is that such sealants, though effective, tend to be quite brittle and are easily fractured in assembly, transportation, or use. When such a fuel cell assembly is used in a relatively high vibrational environment, for example, as an auxiliary power unit (APU) in a land vehicle, the environmental vibration may be sufficient to cause the seals to crack.
Typically, glass seals require high-temperature sintering during manufacture of a fuel cell system, during which the glass devitrifies and flows to fill the interface between the components to bond and seal them. This seal can function satisfactorily until the stack assembly is thermally cycled. Because of differences in the coefficients of thermal expansion (CTE) of the components and the glass seals, and because the glass may progressively crystallize, the seal may fracture, resulting in gas leakage and failure of the fuel cell stack assembly. As the leak increases progressively, cell output diminishes until the total voltage output is unacceptably low.
What is needed is a material for gasketing in an SOFC system which is thermally stable over the range between shutdown and operating temperatures for both the reformer and the fuel cell assembly; which is chemically stable in oxidizing and reducing environments; which is acceptably rugged for assembly and operation of the system; which can provide a dielectric function; and which is compatible with other materials of the system.
It is a principal object of the present invention to provide an improved material for gasketing joints and seals in a fuel cell assembly.